We checked out Amazon's new skyscraper HQ — including the giant glass spheres

In November 2016, Amazon opened "Day 1" — a 521-foot-tall skyscraper in the heart of downtown Seattle that will serve as its new base of operations.

The most well-known feature of the Day 1 tower is actually outside: A set of three gigantic, glass spheres that will be filled with greenery, including endangered species, when they are completed in 2018.

The idea is to give Amazon employees something of an oasis in the middle of the company's famously demanding corporate culture.

I made a return visit in early May while in Seattle for the Microsoft Build conference. There's been a good deal of progress and the project looks impressive. Here's a look at the Day 1 tower and the mysterious glass biospheres that have sprouted up at its base:

This is Day 1 — Amazon's new skyscraper headquarters, which opened in November 2016. It's 37 stories tall. The name refers to Amazon CEO Jeff Bezos mantra that "it's always day 1;" a reminder to never rest on your laurels.

Wikimedia Commons

As you approach the Day 1 tower, you may notice a message on its side. From up close, it's kind of hard to make out...

Matt Weinberger/Business Insider

...but if you step away, you can see that it says "Hello World," a reference to the classic example used to demonstrate new programming languages or concepts.

Matt Weinberger/Business Insider

The building also houses Amazon Go, the retailer's version of a convenience store of the future. Right now, it's only open to Amazon employees, with no timeline for opening to the public.

What makes the store so special is that there are no cash registers or lines — it uses cutting-edge computer vision and payments technology so that you can just walk out with whatever you want.

Amazon

But far and away, the most impressive part of the Day 1 campus are the famous Spheres: Three gigantic glass domes that will be filled full of exotic and endangered flora.

Matt Weinberger/Business Insider

Not to put too fine a point on it, but they're huge. They range from 80 to 90 feet tall.

Matt Weinberger/Business Insider

The domes are impressively massive. Here are some workers on the outside of the dome, for scale.

Matt Weinberger/Business Insider

Last time we visited, in the summer of 2016, this one on the end hadn't had all of its glass installed. Now, things are looking good.

Matt Weinberger/Business Insider

It's difficult to see with the mirrored sides, but if you squint, you can see that Amazon has started to fill the sphere with its first few plants.

Matt Weinberger/Business Insider

(Here's an official Amazon photo of the first plant going into the soil from the inside.)

Amazon

When they open in 2018, the domes are intended to be the center of Amazon employee life. Outside the domes, you can already see some of the green space and weather-shielding canopies already in place. The domes are only for employees, but these areas are for anybody.

Matt Weinberger/Business Insider

And the sphere-area dog park is already in full operation.

Matt Weinberger/Business Insider

And in case you were wondering about the internals of the Day 1 tower, here's a look inside, courtesy of Amazon. This is the lobby area, when you first walk in:

Amazon

There's an airy common area for Amazonians to share a cup of coffee, featuring the backside of the "Hello World" sign.

Amazon

Notice how the lighting is all done up in Amazon black-and-orange.

Amazon

Amazon tries to put some green space inside the building, too.

Amazon

The hallways are austere and modern.

Amazon

Finally, in case you were wondering, the name "Day 1" itself refers to Amazon CEO Jeff Bezos' famous philosophy about never resting on your laurels: "Staying in Day 1 requires you to experiment patiently, accept failures, plant seeds, protect saplings, and double down when you see customer delight."